Vehicle suspension systems must adapt from static conditions of the vehicle at rest to dynamic conditions imposed by travel on a road surface, such as road slope, pitch and turns, external forces like wind gusts, vehicle speed, and load shifts. To handle such conditions, many vehicles have pre-set, fixed suspension parameters for toe, caster, and camber.
The plane of a wheel is the plane perpendicular to the rotation axis of the wheel, passing through the center of the ground contact area of a tire. The term “camber” means the angle formed by the wheel plane relative to a line perpendicular to the ground. Camber of a wheel is also described as the vertical tilt of a wheel either toward or away from the vehicle center. A vehicle with transversely opposed wheels has negative camber when the plane of the wheel tilts outward away from the vehicle when measured at the bottom of the wheel, i.e., the bottom of the wheel is further from the vehicle center than a top of the wheel. Correspondingly, the vehicle has positive camber when the bottom of the wheel is tilted inward toward the vehicle i.e., the bottom of the wheel is nearer to the vehicle center than a top of the wheel. Changes in the camber of a wheel may be described as negative, positive or zero.
Toe is the rotation of the paired wheels either together or apart about their steering axis, that orientation being both vertical and longitudinal relative to the vehicle. A toe-in condition occurs when the fronts of the wheels are both steered inward. A toe-out condition occurs when the fronts of the wheels are both steered outward. Caster angle is the angle of a line drawn between the upper and lower ball joints between the pivot line of the tire and vertical. Caster is negative when the caster angle is >90% and positive when it is <90% when the wheel is viewed from the side.
The term “steering” means the rotation of the wheel plane about the steering axis, which is a line drawn between the upper and lower ball joints and extends to the ground as viewed from the front or rear of the vehicle. The term “roll” is the inclination of the vehicle body about a horizontal axis of the vehicle.
Changing the wheel plane determines the position of the tire on the ground and the stresses imposed on the tire. Further, it determines whether the tire is in the best position for the transmission of forces. Transverse forces are an important force determining the handling of a vehicle.
When a vehicle is steered, the centrifugal forces cause the vehicle's body to roll. When a vehicle is in a “roll” condition, centrifugal force results in a load transfer towards the outside of the curve. This causes compression of the suspension of the outer wheels and a corresponding extension of the suspension of the inner wheels. In conventional unequal length A-arm suspensions, roll imparts negative camber to the outer wheels.
The term “bump steer” is the tendency of a wheel to steer by changing toe, camber or both as it moves upwards into jounce. Generally, the tire that is outside in a turn moves outward as the suspension is compressed, and has a more negative camber. Toe changes may also result from a combination of turning and body roll. In automobiles that have independent unequal length A-arm front suspension, the geometry that controls the camber of the front wheels is a compromise between the need to minimize bump steer and camber change while in jounce modes on the one hand, and the need to create a negative camber change while the front wheels are in turning positions. Since conventional front suspension geometry is designed to compromise these two states of operation, the front wheels produce bump steer when encountering a bump through camber change. Also in a conventional unequal length independent A-arm suspension, if the vehicle's design incorporates a variable ride height, the camber changes as the ride height changes. This provides suboptimal wheel camber because the rights wheel's camber provides a left steering effect while the left wheel provides a right steering effect. Thus, as each front wheel in a currently designed unequal length a-arm suspension encounters uneven road surfaces, each wheel imparts steering effects as they travel in jounce and rebound. These actions decrease vehicle directional stability and increase tire scrub that both reduces tire life and fuel economy.
The present inventor recognizes that it is desirable to have a vehicle suspension that provides a near-zero change in wheel camber while operating in a straight ahead condition. Also, the present inventor recognizes that it is desirable to have a suspension system that dynamically varies the wheel camber during a turn to provide optimal suspension geometry for vehicle stability and performance. Further, the inventor recognizes that it is desirable to have a suspension system that optimally adjusts suspension geometry to account for variations in ride height on vehicles with variable height systems.